1. Field of the Invention
The present invention relates generally to a method of forming coating layers, and more particularly to a method of forming coating layers in which a continuously-running web is coated with multiple PVA coating liquids containing polyvinyl alcohol (PVA) and then the coating liquid layers on the web is dried to form coating layers in the manufacture of heat-sensitive recording paper, pressure-sensitive recording paper, printing plates, or the like.
2. Description of Related Art
PVA is used as a hydrophilic binder when a web is coated with a chemical, and is used for maintaining barrier and surface strength, and the like.
However, it is difficult to stably coat the web with a PVA coating liquid containing PVA with uniform thickness, and it is difficult to form a coating layer with a uniform thickness (uniform drying) by stably and quickly drying the coating liquid layer on the web without making the coating liquid layer rough. This is because the PVA coating liquid does not have property of stabilizing the coating liquid layer by turning into gel at a low temperature like a gelatin solution, and the coating liquid layer gets rough due to a drying wind at the drying and the uniform drying can not be easily performed. In case the web is coated with multiple PVA coating liquids, the interfaces between the layers easily get rough due to the drying wind and be mixed, and thus the uniform drying can not be easily performed. Therefore, the quality of the product is low.
The viscosity of the PVA coating liquid may be raised to prevent the coating liquid layer from getting rough due to the drying wind. But, the stable coating can not be performed when the viscosity is high at the coating even though the coating liquid layer does not get rough if the viscosity is high. Meanwhile, if the viscosity of the PVA coating liquid is adjusted for the coating, the viscosity is so low at the drying that the uniform drying can not be performed. Since the conditions for the stable coating and the uniform drying are directly opposed by each other, it is difficult to satisfy both of them.
A variety of measures has been taken to achieve the stable coating and the uniform drying of the PVA coating liquid.
Japanese Patent Provisional Publication No. 9-164758 discloses a production of ink-jet recording paper comprising the step of adding carrageenan to a PVA coating liquid, and the step of adding 10 millimoles of metal ions or more with respect to a liter of the coating liquid and the step of cooling the PVA coating liquid by lowering the temperature of the PVA coating liquid to the temperature that is 5.degree. C. lower than that at the coating or lower. Japanese Patent Provisional Publication No. 6-67330 discloses a production of photographic material in which 10 millimoles of polysaccharide or more are added with respect to a liter of the coating liquid. Japanese Patent Provisional Publication No. 6-75318 discloses silver halide emulsion including natural polymer red algae polysaccharide (such as carrageenan). Japanese Patent Provisional Publication No. 7-120880 discloses a production of a photographic support in which there are multiple layers including a layer that has polysaccharide that turns into gel when it is cooled. Japanese Patent Provisional Publication No. 8-272014 discloses a production of photographic material in which the web passes a cooling zone that includes red algae polysaccharide and whose temperature is lower than the temperatures of set points of the layers. Japanese Patent Provisional Publication No. 6-81297 discloses that carboxymethel starch, water soluble cellulose or marine algae polysaccharide is added to PVA to improve the coating for paper. Japanese Patent Provisional Publication No. 5-1198 discloses water soluble film composed of PVA to which polysaccharide is added so that the density of the polysaccharide is within the range between 1 and 40% with respect to PVA. Japanese Patent Provisional Publication No. 7-18599 discloses that PVA density and coating temperature are extremely high. The PVA density is within the range between 12 and 60%, and the coating temperature is within the range between 60 and 95.degree. C.
However, the above technics are not good enough for the stable coating and the uniform drying, especially for the uniform drying.
Polysaccharide is added to the PVA coating liquid to give the PVA coating liquid property of turning into gel at a low temperature, but effective property can not always be obtained when polysaccharide is added.
Also, the web is coated with a PVA coating liquid containing polysaccharide at a high temperature, and then the viscosity of the PVA coating liquid is raised with a cold wind to satisfy both the stable coating and the uniform drying. It does not solve the problem since it requires a heat-retaining device for the coating liquid and a product such as a heat-sensitive recording material receives a bad influence from high temperature.
Enough conditions for satisfying both the stable coating and the uniform drying have not been determined, and it is important to determine the conditions.